A particular difficulty in the dispensing or pipetting of very small quantities of liquid concerns errors caused because parts of the liquid to be dosed or metered stick to the pipette probe and are not ejected. Errors of this kind are negligible when quantities are fairly large but become more serious in proportion as the quantities to be dosed or metered are smaller. Tiny drops sticking to the tip of the pipette probe cause considerable difficulties in modern analysis systems working with quantities of liquid of less than 30 .mu.l.
This is a familiar problem and many endeavors have been made to exclude this source of errors in the pipetting of very small quantities of liquid.
Unfortunately, all the proposed solutions are either insufficiently reliable or need very costly apparatus and in some cases there are contamination problems too.